Heretofore, in a water area or the like of a bay, port or the like or around an island, a closed sea area is formed where new sea water no longer or little has been exchanged for old sea water. When contaminated or polluted water flows in such a closed water area from river or drainage or the like, such water remains still in the closed water area, thereby worsening a water quality in the closed sea area as time lapses.
There have been developed various methods for cleaning contaminated or polluted water, for example, as disclosed in Japanese Patent Unexamined Publication No. 6-146,249.
The method for cleaning the water quality of such contaminated or polluted water involves forming a flow path in the closed sea area by mounting both side walls with a plurality of artificial roughness formed at intervals extending in a flow direction and allowing the artificial roughness to create a tidal current flowin one direction within the closed sea area, thereby causing the polluted water flown in the closed sea area to outflow outside the closed sea area and cleaning the water within the closed sea area.
The above-mentioned water quality cleaning method still suffers from the defects as follows:
(1) Where the flow path of the closed sea area is wide, the artificial roughness formed on the side walls thereof can create no tidal oscillation flow. PA1 (2) A mean flow of the tidal oscillation current formed is a one-way flow extending in the flow path of the closed sea area, however, the flow direction cannot be controlled freely in an optional direction. PA1 (3) A mean flow of the one-way tidal oscillation flow formed in the flow path of the closed sea area is a two-dimensional change, not a three-dimensional change that can, for example, prevent the formation of a stratifying phenomenon in which a light surface layer having higher temperature and a heavy deep layer having lower temperature (a layer in a non-oxygen state or a poor-oxygen state) are formed, or destroying the layer.